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Abstract

Creatine is one of the most popular and widely researched natural supplements. The
majority of studies have focused on the effects of creatine monohydrate on performance
and health; however, many other forms of creatine exist and are commercially available
in the sports nutrition/supplement market. Regardless of the form, supplementation
with creatine has regularly shown to increase strength, fat free mass, and muscle
morphology with concurrent heavy resistance training more than resistance training
alone. Creatine may be of benefit in other modes of exercise such as high-intensity
sprints or endurance training. However, it appears that the effects of creatine diminish
as the length of time spent exercising increases. Even though not all individuals
respond similarly to creatine supplementation, it is generally accepted that its supplementation
increases creatine storage and promotes a faster regeneration of adenosine triphosphate
between high intensity exercises. These improved outcomes will increase performance
and promote greater training adaptations. More recent research suggests that creatine
supplementation in amounts of 0.1 g/kg of body weight combined with resistance training
improves training adaptations at a cellular and sub-cellular level. Finally, although
presently ingesting creatine as an oral supplement is considered safe and ethical,
the perception of safety cannot be guaranteed, especially when administered for long
period of time to different populations (athletes, sedentary, patient, active, young
or elderly).

Introduction

Creatine is produced endogenously at an amount of about 1 g/d. Synthesis predominately
occurs in the liver, kidneys, and to a lesser extent in the pancreas. The remainder
of the creatine available to the body is obtained through the diet at about 1 g/d
for an omnivorous diet. 95% of the bodies creatine stores are found in the skeletal
muscle and the remaining 5% is distributed in the brain, liver, kidney, and testes
[1]. As creatine is predominately present in the diet from meats, vegetarians have lower
resting creatine concentrations [2].

Creatine is used and researched in a clinical setting to investigate various pathologies
or disorders such as myopathies [3,4] and is also used as an ergogenic aid for improving health and sports performance
in athletes [5]. As an oral supplement, the most widely used and researched form is creatine monohydrate
(CM). When orally ingested, CM has shown to improve exercise performance and increase
fat free mass [5-9].

There is a great amount of research published on creatine supplementation; protocols
of administration, forms of creatine, as well as potential side effects. Despite this,
the mechanisms by which creatine acts in the human body to improve physical and cognitive
performance are still not clear. The main objectives of this review are to analyze
the more recent findings on the effects and mechanisms of creatine supplementation
in sports and health. As a secondary purpose, we will analyze the most recommended
protocols of ingestion and its potential side effects.

Creatine metabolism

The majority of creatine in the human body is in two forms, either the phosphorylated
form making up 60% of the stores or in the free form which makes up 40% of the stores.
The average 70 kg young male has a creatine pool of around 120-140 g which varies
between individuals [10,11] depending on the skeletal muscle fiber type [1] and quantity of muscle mass [11]. The endogenous production and dietary intake matches the rate of creatinine production
from the degradation of phosphocreatine and creatine at 2.6% and 1.1%/d respectively.
In general, oral creatine supplementation leads to an increase of creatine levels
within the body. Creatine can be cleared from the blood by saturation into various
organs and cells or by renal filtration [1].

Three amino acids (glycine, arginine and methionine) and three enzymes (L-arginine:glycine
amidinotransferase, guanidinoacetate methyltransferase and methionine adenosyltransferase)
are required for creatine synthesis. The impact creatine synthesis has on glycine
metabolism in adults is low, however the demand is more appreciable on the metabolism
of arginine and methionine [11].

Creatine ingested through supplementation is transported into the cells exclusively
by CreaT1. However, there is another creatine transporter Crea T2, which is primarily
active and present in the testes [12]. Creatine uptake is regulated by various mechanisms, namely phosphorylation and glycosylation
as well as extracellular and intracellular levels of creatine. Crea T1 has shown to
be highly sensitive to the extracellular and intracellular levels being specifically
activated when total creatine content inside the cell decreases [12]. It has also been observed that in addition to cytosolic creatine, the existence
of a mitochondrial isoform of Crea T1 allows creatine to be transported into the mitochondria.
Indicating another intra-mitochondrial pool of creatine, which seems to play an essential
role in the phosphate-transport system from the mitochondria to the cytosol [13]. Myopathy patients have demonstrated reduced levels of total creatine and phosphocreatine
as well as lower levels of CreaT1 protein, which is thought to be a major contributor
to these decreased levels [14].

The majority of studies focusing on creatine supplementation report an increase in
the body’s’ creatine pool [15-17]. There is a positive relationship between muscle creatine uptake and exercise performance
[17]. Volek et al [18] observed a significant increase in strength performance after 12 weeks creatine supplementation
with a concurrent periodized heavy resistance training protocol. The creatine supplementation
protocol consisted of a weeklong loading period of 25 g/d followed by a 5 g maintenance
dose for the remainder of the training. These positive effects were attributed to
an increased total creatine pool resulting in more rapid adenosine triphosphate (ATP)
regeneration between resistance training sets allowing athletes to maintain a higher
training intensity and improve the quality of the workouts along the entire training
period.

It is regularly reported that creatine supplementation, when combined with heavy resistance
training leads to enhanced physical performance, fat free mass, and muscle morphology
[18-22]. A 2003 meta analysis [8] showed individuals ingesting creatine, combined with resistance training, obtain
on average +8% and +14% more performance on maximum (1RM) or endurance strength (maximal
repetitions at a given percent of 1RM) respectively than the placebo groups. However,
contradicting studies have reported no effects of creatine supplementation on strength
performance. Jakobi et al [23] found no effects of a short term creatine loading protocol upon isometric elbow flexion
force, muscle activation, and recovery process. However, this study did not clearly
state if creatine supplementation was administered concurrent with resistance training.
Bemben et al [24] have shown no additional benefits of creatine alone or combined with whey protein
for improving strength and muscle mass after a progressive 14 weeks (3 days per week)
resistance training program in older men. These conflicting results can be explained
by the possibility that the supplemented groups were formed by a greater amount of
non-responders or even because creatine supplementation was administered on the training
days only (3 times a week). This strategy has not been adequately tested as effective
in middle aged and older men for maintaining post loading elevated creatine stores
[5].

A quantitative, comprehensive scientific summary and view of knowledge up to 2007
on the effects of creatine supplementation in athletes and active people was published
in a 100 citation review position paper by the International Society of Sports Nutrition[5]. More recent literature has provided greater insight into the anabolic/performance
enhancing mechanisms of creatine supplementation [15,25] suggesting that these effects may be due to satellite cell proliferation, myogenic
transcription factors and insulin-like growth factor-1 signalling [16]. Saremi et al [26] reported a change in myogenic transcription factors when creatine supplementation
and resistance training are combined in young healthy males. It was found that serum
levels of myostatin, a muscle growth inhibitor, were decreased in the creatine group.

Collectively, in spite of a few controversial results, it seems that creatine supplementation
combined with resistance training would amplify performance enhancement on maximum
and endurance strength as well muscle hypertrophy.

Creatine has demonstrated neuromuscular performance enhancing properties on short
duration, predominantly anaerobic, intermittent exercises. Bazzucch et al [27] observed enhanced neuromuscular function of the elbow flexors in both electrically
induced and voluntary contractions but not on endurance performance after 4 loading
doses of 5 g creatine plus 15 g maltodextrin for 5/d in young, moderately trained
men. Creatine supplementation may facilitate the reuptake of Ca2+ into the sacroplasmic reticulum by the action of the Ca2+ adenosine triphosphatase pump, which could enable force to be produced more rapidly
through the faster detachment of the actomyosin bridges.

A previous meta-analysis [28] reported an overall creatine supplementation effect size (ES) of 0.24 ± 0.02 for
activities lasting ≤30 s. (primarily using the ATP- phosphocreatine energy system).
For this short high-intensity exercise, creatine supplementation resulted in a 7.5 ± 0.7%
increase from base line which was greater than the 4.3 ± 0.6% improvement observed
for placebo groups. When looking at the individual selected measures for anaerobic
performance the greatest effect of creatine supplementation was observed on the number
of repetitions which showed an ES of 0.64 ± 0.18. Furthermore, an increase from base
line of 45.4 ± 7.2% compared to 22.9 ± 7.3% for the placebo group was observed. The
second greatest ES was on the weight lifted at 0.51 ± 0.16 with an increase from base
line of 13.4 ± 2.7% for the placebo group and 24.7 ± 3.9% for the creatine group.
Other measures improved by creatine with a mean ES greater than 0 were for the amount
of work accomplished, weight lifted, time, force production, cycle ergometer revolutions/min
and power. The possible effect of creatine supplementation on multiple high intensity
short duration bouts (<30 s) have shown an ES not statistically significant from 0.
This would indicate that creatine supplementation might be useful to attenuate fatigue
symptoms over multiple bouts of high-intensity, short duration exercise. The ES of
creatine on anaerobic endurance exercise (>30 – 150s), primarily using the anaerobic
glycolysis energy system, was 0.19 ± 0.05 with an improvement from baseline of 4.9 ± 1.5
% for creatine and -2.0 ± 0.6% for the placebo. The specific aspects of anaerobic
endurance performance improved by creatine supplementation were work and power, both
of which had a mean ES greater than 0. From the findings of this previous meta-analysis
[28] it would appear that creatine supplementation has the most pronounced effect on short
duration (<30s) high intensity intermittent exercises.

Effects of creatine supplementation on skeletal muscle hypertrophy

Cribb et al (2007) [29] observed greater improvements on 1RM, lean body mass, fiber cross sectional area
and contractile protein in trained young males when resistance training was combined
with a multi-nutrient supplement containing 0.1 g/kg/d of creatine, 1.5 g/kg/d of
protein and carbohydrate compared with protein alone or a protein carbohydrate supplement
without the creatine. These findings were novel because at the time no other research
had noted such improvements in body composition at the cellular and sub cellular level
in resistance trained participants supplementing with creatine. The amount of creatine
consumed in the study by Cribb et al was greater than the amount typically reported
in previous studies (a loading dose of around 20 g/d followed by a maintenance dose
of 3-5 g/d is generally equivalent to approximately 0.3 g/kg/d and 0.03 g/kg/d respectively)
and the length of the supplementation period or absence of resistance exercise may
explain the observed transcriptional level changes that were absent in previous studies
[30,31].

Deldicque et al [32] found a 250%, 45% and 70% increase for collagen mRNA, glucose transporter 4 (GLUT4)
and Myosin heavy chain IIA, respectively after 5 days creatine loading protocol (21
g/d). The authors speculated that creatine in addition to a single bout of resistance
training can favor an anabolic environment by inducing changes in gene expression
after only 5 days of supplementation.

When creatine supplementation is combined with heavy resistance training, muscle insulin
like growth factor (IGF-1) concentration has been shown to increase. Burke et al [2] examined the effects of an 8 week heavy resistance training protocol combined with
a 7 day creatine loading protocol (0.25 g/d/kg lean body mass) followed by a 49 day
maintenance phase (0.06 g/kg lean mass) in a group of vegetarian and non-vegetarian,
novice, resistance trained men and women. Compared to placebo, creatine groups produced
greater increments in IGF-1 (78% Vs 55%) and body mass (2.2 Vs 0.6 kg). Additionally,
vegetarians within the supplemented group had the largest increase of lean mass compared
to non vegetarian (2.4 and 1.9 kg respectively). Changes in lean mass were positively
correlated to the modifications in intramuscular total creatine stores which were
also correlated with the modified levels of intramuscular IGF-1. The authors suggested
that the rise in muscle IGF-1 content in the creatine group could be due to the higher
metabolic demand created by a more intensely performed training session. These amplifying
effects could be caused by the increased total creatine store in working muscles.
Even though vegetarians had a greater increase in high energy phosphate content, the
IGF-1 levels were similar to the amount observed in the non vegetarian groups. These
findings do not support the observed correlation pattern by which a low essential
amino acid content of a typical vegetarian diet should reduce IGF-1 production [33]. According to authors opinions it is possible that the addition of creatine and subsequent
increase in total creatine and phosphocreatine storage might have directly or indirectly
stimulated production of muscle IGF-I and muscle protein synthesis, leading to an
increased muscle hypertrophy [2].

Effects of creatine supplementation on predominantly aerobic exercise

Although creatine supplementation has been shown to be more effective on predominantly
anaerobic intermittent exercise, there is some evidence of its positive effects on
endurance activities. Branch [28] highlights that endurance activities lasting more than 150s rely on oxidative phosphorylation
as primary energy system supplier. From this meta analysis [28], it would appear that the ergogenic potential for creatine supplementation on predominantly
aerobic endurance exercise diminishes as the duration of the activity increases over
150s. However it is suggested that creatine supplementation may cause a change in
substrate utilization during aerobic activity possibly leading to an increase in steady
state endurance performance.

Chwalbinska-Monteta [34] observed a significant decrease in blood lactate accumulation when exercising at
lower intensities as well as an increase in lactate threshold in elite male endurance
rowers after consuming a short loading (5 days 20 g/d) CM protocol. However, the effects
of creatine supplementation on endurance performance have been questioned by some
studies. Graef et al [35] examined the effects of four weeks of creatine citrate supplementation and high-intensity
interval training on cardio respiratory fitness. A greater increase of the ventilatory
threshold was observed in the creatine group respect to placebo; however, oxygen consumption
showed no significant differences between the groups. The total work presented no
interaction and no main effect for time for any of the groups. Thompson et al [36] reported no effects of a 6 week 2 g CM/d in aerobic and anaerobic endurance performance
in female swimmers. In addition, of the concern related to the dosage used in these
studies, it could be possible that the potential benefits of creatine supplementation
on endurance performance were more related to effects of anaerobic threshold localization.

Effects of creatine supplementation on glycogen stores

It is suggested [16,37] that another mechanism for the effect of creatine could be enhanced muscle glycogen
accumulation and GLUT4 expression, when creatine supplementation is combined with
a glycogen depleting exercise. Whereas it has been observed [38] that creatine supplementation alone does not enhance muscle glycogen storage. Hickner
et al [15] observed positive effects of creatine supplementation for enhancing initial and maintaining
a higher level of muscle glycogen during 2 hours of cycling. In general, it is accepted
that glycogen depleting exercises, such as high intensity or long duration exercise
should combine high carbohydrate diets with creatine supplementation to achieve heightened
muscle glycogen stores [39].

Creatine supplementation may also be of benefit to injured athletes. Op’t Eijnde et
al [39] noted that the expected decline in GLUT4 content after being observed during a immobilization
period can be offset by a common loading creatine (20g/d) supplementation protocol.
In addition, combining CM 15g/d for 3 weeks following 5 g/d for the following 7 weeks
positively enhances GLUT4 content, glycogen, and total muscle creatine storage [39].

Cooke et al [41] observed positive effects of a prior (0.3 g/d kg BW) loading and a post maintenance
protocol (0.1 g/d kg BW) to attenuate the loss of strength and muscle damage after
an acute supramaximal (3 set x 10 rep with 120% 1RM) eccentric resistance training
session in young males. The authors speculate that creatine ingestion prior to exercise
may enhance calcium buffering capacity of the muscle and reduce calcium-activated
proteases which in turn minimize sarcolemma and further influxes of calcium into the
muscle. In addition creatine ingestion post exercise would enhance regenerative responses,
favoring a more anabolic environment to avoid severe muscle damage and improve the
recovery process. In addition, in vitro studies have demonstrated the antioxidant
effects of creatine to remove superoxide anion radicals and peroxinitrite radicals
[42]. This antioxidant effect of creatine has been associated with the presence of Arginine
in its molecule. Arginine is also a substrate for nitric oxide synthesis and can increase
the production of nitric oxide which has higher vasodilatation properties, and acts
as a free radical that modulates metabolism, contractibility and glucose uptake in
skeletal muscle. Other amino acids contained in the creatine molecule such as glycine
and methinine may be especially susceptible to free radical oxidation because of sulfhydryl
groups [42]. A more recent in vitro study showed that creatine exerts direct antioxidant activity
via a scavenging mechanism in oxidatively injured cultured mammalian cells [43]. In a recent in vivo study Rhaini et al [44] showed a positive effect of 7 days of creatine supplementation (4 x 5 g CM 20 g total)
on 27 recreational resistance trained males to attenuate the oxidation of DNA and
lipid peroxidation after a strenuous resistance training protocol.

Collectively the above investigations indicate that creatine supplementation can be
an effective strategy to maintain total creatine pool during a rehabilitation period
after injury as well as to attenuate muscle damage induced by a prolonged endurance
training session. In addition, it seems that creatine can act as an effective antioxidant
agent after more intense resistance training sessions.

Effects of creatine supplementation on range of motion

Sculthorpe et al (2010) has shown that a 5 day (25g/d) loading protocol of creatine
supplementation followed by a further 3 days of 5 g/d negatively influence both active
ankle dorsiflexion and shoulder abduction and extension range of movement (ROM) in
young men. There are two possible theories to explain these effects: 1) Creatine supplementation
increases intracellular water content resulting in increased muscle stiffness and
resistance to stretch; 2) Neural outflow from the muscle spindles is affected due
to an increased volume of the muscle cell. The authors highlight that the active ROM
measures were taken immediately after the loading phase and the reduced active ROM
may not be seen after several weeks of maintenance phase [45]. Hile et al [46] observed an increase in compartment pressure in the anterior compartment of the lower
leg, which may also have been responsible for a reduced active ROM.

Documented effects of creatine supplementation for health and clinical setting

Neurological and cognitive function has also been shown to be improved by creatine
supplementation [47,48]. Rawson and Venezia [49] review the effects of creatine supplementation on cognitive function highlighting
that higher brain creatine has been associated with improved neuropsychological performance.
Creatine supplementation protocols have been shown to increase brain creatine and
phosphocreatine contents. Cognitive processing hindered due to sleep deprivation and
natural impairment due to aging can be improved by creatine supplementation. This
review also highlights other possible benefits of creatine ingestion to older adults,
such as improvements in: fatigue resistance, strength, muscle mass, bone mineral density,
and performance of activities of daily living. Some of these benefits occur without
concurrent exercise. The authors inform that discrepancies between studies do exist
and are hard to explain but may be possibly due to differences in diet, race and/or
supplementation protocols. However, the ideal dose of creatine to maximize brain uptake
is not known. Patients have been supplemented with 40 g while in healthy adults positive
results have been reported with around 20 g per day [49].

Studies with animal and cellular models demonstrated positive effect of creatine ingestion
on neurodegenerative diseases. These effects have been attributed to improved overall
cellular bioenergetics due to an expansion of the phosphocreatine pool [50]. Creatine deficiency syndromes, due to deficiency of glycine amidinotransferase and
guanidinoacetate methyltransferase, can cause decreases or complete absence of creatine
in the central nervous system. Syndromes of this nature have the possibility to be
improved by supplementing orally with creatine. Brain creatine deficiency resulting
from ineffective crea T1 has been shown not to be effectively treated with oral creatine
supplementation [51]. Additionally, oral creatine administration in patients with myopathies has shown
conflicting results depending on the type of myopathy and creatine transport systems
disorders [4].

Creatine use in children and adolescents

Creatine supplementation in the under 18 population has not received a great deal
of attention, especially in regards to sports/exercise performance. Despite this,
creatine is being supplemented in young, <18 years old, athletes [52,53]. In a 2001 report [52] conducted on pupils from middle and high school (aged 10 – 18) in Westchester County
(USA) 62 of the 1103 pupils surveyed were using creatine. The authors found this concerning
for 2 main reasons: firstly, the safety of creatine supplementation is not established
for this age group and is therefore not recommended. Secondly, it was speculated that
taking creatine would lead on to more dangerous performance enhancing products such
as anabolic steroids. It is important to point out that this potential escalation
is speculation. Furthermore, a questionnaire was used to determine creatine use amongst
this age group and does not necessarily reflect the truth.

A child’s ability to regenerate high energy phosphates during high intensity exercise
is less than that of an adult. Due to this, creatine supplementation may benefit the
rate and use of creatine phosphate and ATP rephosporylation. However, performance
in short duration high-intensity exercise can be improved through training therefore
supplementation may not be necessary [54].

Based on the limited data on performance and safety, some authors have not identified
any conclusions and do not recommend its consumption in regards to creatine supplementation
in children and adolescents [52,54]. Conversely, according to the view of the ISSN [5], younger athletes should consider a creatine supplement under certain conditions:
puberty is past and he/she is involved in serious competitive training; the athlete
is eating a well-balanced caloric adequate diet; he/she as well as the parents approve
and understand the truth concerning the effects of creatine supplementation; supplement
protocols are supervised by qualified professionals; recommended doses must not be
exceeded; quality supplements are administered.

Within this framework, creatine supplementation in young, post puberty athletes can
be considered a high quality type of “food” that can offer additional benefits to
optimise training outcomes.

Dosing protocols applied in creatine supplementation

A typical creatine supplementation protocol consists of a loading phase of 20 g CM/d
or 0.3 g CM/kg/d split into 4 daily intakes of 5 g each, followed by a maintenance
phase of 3-5 g CM/d or 0.03 g CM/kg/d for the duration of the supplementation period
[5]. Other supplementation protocols are also used such as a daily single dose of around
3 – 6 g or between 0.03 to 0.1 g/kg/d [15,55] however this method takes longer (between 21 to 28 days) to produce ergogenic effects
[5]. Sale et al [56] found that a moderate protocol consisting of 20 g CM taken in 1g doses (evenly ingested
at 30-min intervals) for 5 days resulted in reduced urinary creatine and methylamine
excretion, leading to an estimated increase in whole body retention of creatine (+13%)
when compared with a typical loading supplementation protocol of 4 x 5 g/d during
5 days (evenly ingested at 3 hour intervals). This enhancement in creatine retention
would lead to a significantly higher weight gain when people follow a moderate protocol
ingestion of several doses of small amounts of CM evenly spread along the day.

Responders vs. non-responders

Syrotuik and Bell [57] investigated the physical characteristics of responder and non-responder subjects
to creatine supplementation in recreationally resistance trained men with no history
of CM usage. The supplement group was asked to ingest a loading dosage of 0.3 g/kg/d
for 5 days. The physiological characteristics of responders were classified using
Greenhaff et al [58] criterion of >20 mmol/kg dry weight increase in total intramuscular creatine and
phosphocreatine and non responders as <10 mmol/kg dry weight increase, a third group
labeled quasi responders were also used to classify participants who fell in between
the previously mentioned groups (10-20 mmol/kg dry weight). Overall, the supplemented
group showed a mean increase in total resting muscle creatine and phosphocreatine
of 14.5% (from 111.12 ± 8.87 mmol/kg dry weight to 127.30 ± 9.69 mmol/kg dry weight)
whilst the placebo group remained relatively unaffected (from 115.70 ± 14.99 mmol/kg
dry weight to 111.74 ± 12.95 mmol/kg dry weight). However when looking at individual
cases from the creatine group the results showed a variance in response. From the
11 males in the supplemented group, 3 participants were responders (mean increase
of 29.5 mmol/kg dry weight or 27%), 5 quasi responders (mean increase of 14.9 mmol/kg
dry weight or 13.6%) and 3 non-responders (mean increase of 5.1 mmol/kg dry weight
or 4.8%). Using muscle biopsies of the vastus lateralis, a descending trend for groups
and mean percentage fiber type was observed. Responders showed the greatest percentage
of type II fibers followed by quasi responders and non-responders. The responder and
quasi responder groups had an initial larger cross sectional area for type I, type
IIa and type IIx fibers. The responder group also had the greatest mean increase in
the cross sectional area of all the muscle fiber types measured (type I, type IIa
and type IIx increased 320, 971 and 840 μm2 respectively) and non-responders the least (type I, type IIa and type IIx increased
60, 46 and 78 μm2 respectively). There was evidence of a descending trend for responders to have the
highest percentage of type II fibers; furthermore, responders and quasi responders
possessed the largest initial cross sectional area of type I, IIa and IIx fibers.
Responders were seen to have the lowest initial levels of creatine and phosphocreatine.
This has also been observed in a previous study [17] which found that subjects whose creatine levels were around 150 mmol/Kg dry mass
did not have any increments in their creatine saturation due to creatine supplementation,
neither did they experience any increases of creatine uptake, phosphocreatine resynthesis
and performance. This would indicate a limit maximum size of the creatine pool.

In summary responders are those individuals with a lower initial level of total muscle
creatine content, greater population of type II fibers and possess higher potential
to improve performance in response to creatine supplementation.

Commercially available forms of creatine

There are several different available forms of creatine: creatine anhydrous which
is creatine with the water molecule removed in order to increase the concentration
of creatine to a greater amount than that found in CM. Creatine has been manufactured
in salt form: creatine pyruvate, creatine citrate, creatine malate, creatine phosphate,
magnesium creatine, creatine oroate, Kre Alkalyn (creatine with baking soda). Creatine
can also be manufactured in an ester form. Creatine ethyl ester (hydrochloride) is
an example of this, as is creatine gluconate which is creatine bound to glucose. Another
form is creatine effervescent which is creatine citrate or CM with citric acid and
bicarbonate. The citric acid and bicarbonate react to produce an effervescent effect.
When mixed with water the creatine separates from its carrier leaving a neutrally
charged creatine, allowing it to dissolve to a higher degree in water. Manufacturers
claim that creatine effervescent has a longer and more stable life in solution. When
di-creatine citrate effervescent was studied [59] for stability in solution it was found that the di-creatine citrate dissociates to
citric acid and creatine in aqueous solutions which in turn forms CM and eventually
crystallises out of the solution due to its low solubility. Some of the creatine may
also convert to creatinine.

Jager et al [60] observed 1.17 and 1.29 greater peak plasma creatine concentration 1 hour after ingesting
creatine pyruvate compared to isomolar amount of CM and creatine citrate respectively.
However time to peak concentration, and velocity constants of absorption and elimination,
was the same for all three forms of creatine. Although not measured in this study
it is questionable that these small differences in plasma creatine concentrations
would have any effect on the increase of muscle creatine uptake. Jäger et al [61] investigated the effects of 28-days of creatine pyruvate and citrate supplementation
on endurance capacity and power measured during an intermittent handgrip (15 s effort
per 45s rest) exercise in healthy young athletes. The authors used a daily dose protocol
with the intention to slowly saturate muscle creatine stores. Both forms of creatine
showed slightly different effects on plasma creatine absorption and kinetics. The
two creatine salts significantly increased mean power but only pyruvate forms showed
significant effects for increasing force and attenuating fatigability during all intervals.
These effects can be attributed to an enhanced contraction and relaxation velocity
as well as a higher blood flow and muscle oxygen uptake. On the other hand, the power
performance measured with the citrate forms decreases with time and improvements were
not significant during the later intervals. In spite of these positive trends further
research is required about the effects of these forms of creatine as there is little
or no evidence for their safety and efficacy. Furthermore the regularity status of
the novel forms of creatine vary from country to country and are often found to be
unclear when compared to that of CM [62].

In summary, creatine salts have been show to be less stable than CM. However the addition
of carbohydrates could increase their stability [62]. The potential advantages of creatine salts over CM include enhanced aqueous solubility
and bioavailability which would reduce their possible gastrointestinal adverse effects
[63]. The possibility for new additional formulation such as tablets or capsules is interesting
for its therapeutic application due to its attributed better dissolution kinetics
and oral absorption compared to CM [63]. However more complete in vivo pharmaceutical analysis of creatine salts are required
to fully elucidate their potential advantages/disadvantages over the currently available
supplement formulations.

Creatine is a hydrophilic polar molecule that consists of a negatively charged carboxyl
group and a positively charged functional group [64]. The hydrophilic nature of creatine limits its bioavailability [65]. In an attempt to increase creatines bioavailability creatine has been esterified
to reduce the hydrophilicity; this product is known as creatine ethyl ester. Manufacturers
of creatine ethyl ester promote their product as being able to by-pass the creatine
transporter due to improved sarcolemmal permeability toward creatine [65]. Spillane et al [65] analyzed the effects of a 5 days loading protocol (0.30 g/kg lean mass) followed
by a 42 days maintenance phase (0.075 g/kg lean mass) of CM or ethyl ester both combined
with a resistance training program in 30 novice males with no previous resistance
training experience. The results of this study [65] showed that ethyl ester was not as effective as CM to enhance serum and muscle creatine
stores. Furthermore creatine ethyl ester offered no additional benefit for improving
body composition, muscle mass, strength, and power. This research did not support
the claims of the creatine ethyl ester manufacturers.

Polyethylene glycol is a non-toxic, water-soluble polymer that is capable of enhancing
the absorption of creatine and various other substances [66]. Polyethylene glycol can be bound with CM to form polyethylene glycosylated creatine.
One study [67] found that 5 g/d for 28 days of polyethylene glycosylated creatine was capable of
increasing 1RM bench press in 22 untrained young men but not for lower body strength
or muscular power. Body weight also did not significantly change in the creatine group
which may be of particular interest to athletes in weight categories that require
upper body strength. Herda et al [68] analyzed the effects of 5 g of CM and two smaller doses of polyethylene glycosylated
creatine (containing 1.25 g and 2.5 g of creatine) administered over 30 days on muscular
strength, endurance, and power output in fifty-eight healthy men. CM produced a significantly
greater improvement in mean power and body weight meanwhile both CM and polyethylene
glycosylated form showed a significantly (p < 0.05) greater improvement for strength
when compared with control group. These strength increases were similar even though
the dose of creatine in the polyethylene glycosylated creatine groups was up to 75%
less than that of CM. These results seem to indicate that the addition of polyethylene
glycol could increase the absorption efficiency of creatine but further research is
needed before a definitive recommendation can be reached.

Creatine in combination with other supplements

Although creatine can be bought commercially as a standalone product it is often found
in combination with other nutrients. A prime example is the combination of creatine
with carbohydrate or protein and carbohydrate for augmenting creatine muscle retention
[5] mediated through an insulin response from the pancreas [69]. Steenge et al [70] found that body creatine retention of 5 g CM was increased by 25% with the addition
of 50 g of protein and 47 g of carbohydrate or 96 g carbohydrate when compared to
a placebo treatment of 5 g carbohydrate. The addition of 10g of creatine to 75 g of
dextrose, 2 g of taurine, vitamins and minerals, induced a change in cellular osmolarity
which in addition to the expected increase in body mass, seems to produce an up regulation
of large scale gene expression (mRNA content of genes and protein content of kinases
involved in osmosensing and signal transduction, cytoskeleton remodelling, protein
and glycogen synthesis regulation, satellite cell proliferation and differentiation,
DNA replication and repair, RNA transcription control, and cell survival) [25]. Similar findings have also been reported for creatine monohydrate supplementation
alone when combined with resistance training [71].

A commercially available pre-workout formula comprised of 2.05 g of caffeine, taurine
and glucuronolactone, 7.9 g of L-leucine, L-valine, L-arginine and L-glutamine, 5
g of di-creatine citrate and 2.5 g of β-alanine mixed with 500 ml of water taken 10
minutes prior to exercise has been shown to enhance time to exhaustion during moderate
intensity endurance exercise and to increase feelings of focus, energy and reduce
subjective feelings of fatigue before and during endurance exercise due to a synergistic
effect of the before mentioned ingredients [72]. The role of creatine in this formulation is to provide a neuroprotective function
by enhancing the energy metabolism in the brain tissue, promoting antioxidant activities,
improving cerebral vasculation and protecting the brain from hyperosmotic shock by
acting as a brain cell osmolyte. Creatine can provide other neuroprotective benefits
through stabilisation of mitochondrial membranes, stimulation of glutamate uptake
into synaptic vesicles and balance of intracellular calcium homeostasis [72].

Safety and side effects of creatine supplementation

There have been a few reported renal health disorders associated with creatine supplementation
[73,74]. These are isolated reports in which recommended dosages are not followed or there
is a history of previous health complaints, such as renal disease or those taking
nephrotoxic medication aggravated by creatine supplementation [73]. Specific studies into creatine supplementation, renal function and/or safety conclude
that although creatine does slightly raise creatinine levels there is no progressive
effect to cause negative consequences to renal function and health in already healthy
individuals when proper dosage recommendations are followed [73-77]. Urinary methylamine and formaldehyde have been shown to increase due to creatine
supplementation of 20 g/d; this however did not bring the production outside of normal
healthy range and did not impact on kidney function [56,78]. It has been advised that further research be carried out into the effects of creatine
supplementation and health in the elderly and adolescent [73,75]. More recently, a randomized, double blind, 6 month resistance exercise and supplementation
intervention [79] was performed on elderly men and women (age >65 years) in which subjects were assigned
to either a supplement or placebo group. The supplement group was given 5 g CM, 2
g dextrose and 6 g conjugated linoleic acid/d, whilst the placebo group consumed 7
g dextrose and 6 g safflower oil/d. CM administration showed significantly greater
effects to improve muscular endurance, isokinetic knee extension strength, fat free
mass and to reduce fat mass compared to placebo. Furthermore the supplement group
had an increase in serum creatinine but not creatinine clearance suggesting no negative
effect on renal function.

Cornelissen et al [80] analyzed the effects of 1 week loading protocol (3 X 5 g/d CM) followed by a 3 month
maintenance period (5 g/d) on cardiac patients involved in an endurance and resistance
training program. Although CM supplementation did not significantly enhance performance,
markers of renal and liver function were within normal ranges indicating the safety
of the applied creatine supplementation protocol.

A retrospective study [81], that examined the effects of long lasting (0.8 to 4 years) CM supplementation on
health markers and prescribed training benefits, suggested that there is no negative
health effects (including muscle cramp or injuries) caused by long term CM consumption.
In addition, despite many anecdotal claims, it appears that creatine supplementation
would have positive influences on muscle cramps and dehydration [82]. Creatine was found to increase total body water possibly by decreasing the risk
of dehydration, reducing sweat rate, lowering core body temperature and exercising
heart rate. Furthermore, creatine supplementation does not increase symptoms nor negatively
affect hydration or thermoregulation status of athletes exercising in the heat [83,84]. Additionally, CM ingestion has been shown to reduce the rate of perceived exertion
when training in the heat [85].

It is prudent to note that creatine supplementation has been shown to reduce the body’s
endogenous production of creatine, however levels return to normal after a brief period
of time when supplementation ceases [1,6]. Despite this creatine supplementation has not been studied/supplemented with for
a relatively long period. Due to this, long term effects are unknown, therefore safety
cannot be guaranteed. Whilst the long term effects of creatine supplementation remain
unclear, no definitive certainty of either a negative or a positive effect upon the
body has been determined for many health professionals and national agencies [19,78]. For example the French Sanitary Agency has banned the buying of creatine due to
the unproven allegation that a potential effect of creatine supplementation could
be that of mutagenicity and carcinogenicity from the production of heterocyclic amines
[78]. Long term and epidemiological data should continue to be produced and collected
to determine the safety of creatine in all healthy individuals under all conditions
[78].

A typical creatine supplementation protocol of either a loading phase of 20 to 25
g CM/d or 0.3 g CM/kg/d split into 4 to 5 daily intakes of 5 g each have been recommended
to quickly saturate creatine stores in the skeletal muscle. However a more moderate
protocol where several smaller doses of creatine are ingested along the day (20 intakes
of 1 g every 30 min) could be a better approach to get a maximal saturation of the
intramuscular creatine store. In order to keep the maximal saturation of body creatine,
the loading phase must be followed by a maintenance period of 3-5 g CM/d or 0.03 g
CM/kg/d. These strategies appear to be the most efficient way of saturating the muscles
and benefitting from CM supplementation. However more recent research has shown CM
supplementation at doses of 0.1 g/kg body weight combined with resistance training
improves training adaptations at a cellular and sub-cellular level. Creatine retention
by the body from supplementation appears to be promoted by about 25% from the simultaneous
ingestion of carbohydrate and/or protein mediated through an increase in insulin secretion.
This combination would produce a faster saturation rate but has not been shown to
have a greater effect on performance.

Different forms of creatine in combination with other sports supplements as well as
varying doses and supplementation methodology should continue to be researched in
an attempt to understand further application of creatine to increase sports and exercise
performance of varying disciplines. It is important to remain impartial when evaluating
the safety of creatine ingested as a natural supplement. The available evidence indicates
that creatine consumption is safe. This perception of safety cannot be guaranteed
especially that of the long term safety of creatine supplementation and the various
forms of creatine which are administered to different populations (athletes, sedentary,
patient, active, young or elderly) throughout the globe.

Evans MW, Ndetan H, Perko M, Williams R, Walker C: Dietary supplement use by children and adolescents in the United States to Enhance
sport performance: results of the national health interview survey.